Cementitious building materials, such as concrete and gypsum products, are typically prepared by mixing dehydrated inorganic material with water and casting the resulting slurry into molds, forms, or sheets where it hydrates, hardens, and dries. For example, the production of gypsum-containing articles involves combining calcined gypsum powder (calcium sulfate hemihydrate and/or calcium sulfate anhydrite) with water (and often a small percentage of a variety of additives), and casting the mixture into a desired shaped mold or onto a surface. The resulting hydration reaction produces an interlocking matrix of gypsum crystals (calcium sulfate dihydrate). This is often followed by mild heating to drive off the remaining free (unreacted) water to yield a dry product.
Cementitious materials are used universally, primarily in the construction industry, for their desirable qualities of ease of casting, high compressive strength, and fire-resistance. Cementitious products include concrete, lightweight concrete, reinforced concrete, concrete board, gypsum boards, reinforced gypsum composite boards, plasters, machinable materials, joint treatment materials, acoustical tiles, plaster casts, and dental models. The most notable shortcoming is the weight of the products produced using cementitious materials, which results in relatively high production, installation, and building costs. Since the strength of a given composition is proportional to its density, current cementitious building materials must have relatively high densities in order to achieve desired performance requirements. The density of the material, and thus the overall weight of the products, can be reduced by introducing air voids or expanded filler into the inorganic material but only with a loss in strength that is more than merely proportional to the weight loss.
All of the cementitious products described above would benefit from increased strength-to-weight ratio, which would make them more resistant to the stresses encountered during use while reducing weight and building costs. Wallboard, the largest volume gypsum product would particularly benefit from such an improvement. Wallboard typically consists of a gypsum core sandwiched between sheets of cover paper. In an effort to decrease the weight of the product, producers have steadily increased the porosity of the gypsum core by incorporating air voids or lightweight filler. The core is thus weak and the majority of current wallboard strength is provided by fiber-oriented, multi-ply cover paper. Paper is by far the most expensive component of wallboard manufacture, contributing more than 40% to the manufacturing cost. In addition, the paper facing of wallboard is subject to mold, which consumes the cellulosic material, deteriorates the mechanical integrity of the board, and produces foul smelling, toxic chemicals.
There is continuing effort to make gypsum-containing products lighter in weight by substituting lower density materials (e.g., expanded perlite or air voids) for part of their set gypsum matrix. In such compositions, there is a need to increase the strength of the set gypsum above normal levels in order to maintain overall product strength because there is less set gypsum mass to provide strength in the lower density product.
A number of additives, such as cellulosic particles and fibers, have been included to further improve the mechanical properties of cementitious products. More expensive glass fibers are used in place of wood in applications where high fire resistance is required. However, conventional fibers, particularly glass, do not adhere well to the gypsum matrix and decrease the workability of the gypsum slurry, thus limiting improvement of the board. Glass fibers are also brittle and can be easily dislodged during board handling, installation, or demolition to cause irritation of the skin or lungs.
More recently, there has been increasing interest in improving the strength and wear resistance of construction materials by incorporating synthetic polymers. Cementitious composites containing water-dispersible polymers having modest improvement in strength-to-weight have been found by adding latex or other strengthening polymers to the cementitious materials. However, several unique challenges have thus far restricted the commercialization of polymer reinforced cementitious products to relatively expensive niche products.
U.S. Pat. No. 6,402,832 (“the '832 Patent”) describes the use of additives in quick-drying joint compound. In one example, a water soluble functional polymer with either a nitrogen or a sulfonate group, such as poly(vinyl pyrrolidone) (“PVP”) at a molecular weight of between 20,000 and 40,000 (all molecular weights reported herein are in Daltons), was combined with a powdered solid bisphenol-A-based epoxy resin, such as Shell EPON™ 1002F (“Epoxy”), achieving a crack resistance strength slightly higher than PVP alone and a slightly faster drying time than PVP alone.
However, the PVP and Epoxy additives of the '832 Patent, either alone or together, decreased the porosity caused by evaporation of water from the slurry (the '832 Patent, column 4, II. 46-54). According to the '832 Patent, the decrease in porosity of the joint compound was the primary mechanism in the increased crack resistance (the '832 Patent, column 1, II. 50-59), which was based on the load required for crack initiation in the joint between two pieces of wallboard.
In other examples, the '832 Patent taught that a range of molecular weight of between 40,000 and 80,000 for PVP produced significantly improved crack resistance compared to higher molecular weight PVP (the '832 Patent, column 6, II. 3-5). At this molecular weight, a concentration of between 3 wt % and 6 wt % of PVP with between 2 wt % and 4 wt % Epoxy was disclosed as an optimal, lowest range of concentration to achieve an optimally improved combination of both crack resistance and drying times (the '832 Patent, column 7, II. 7-21).
The cost of polymers is typically hundreds of times that of the inorganic material, particularly for gypsum products, and additions of strengthening polymers normally are restricted to a small percentage of the mixture (e.g., less than 1% of weight of stucco for wallboard applications) to be successful commercially. However, because high strength polymers typically have a low adhesion to inorganic materials and tend to coagulate in aqueous solution, large amounts of polymer (or compatibilizers, such as surfactants) are needed to improve the strength to weight ratio of the composites.
Alternatively, hydrophilic polymers adhere well to gypsum crystals but tend to either: (1) have low intrinsic film strengths; (2) bind so well to gypsum crystals that hydration and crystal growth, and thus composite strength, are severely retarded; or (3) show a greater affinity to water than the inorganic material and migrate to the edges of the sample with the evaporating moisture leaving the core without reinforcement and weak.
In U.S. application Ser. No. 10/094,572, filed Mar. 7, 2002, (“the '572 Application”) the specification of which is incorporated herein in its entirety, polymers overcoat the inorganic, filler particles, providing adhesion between the particles and cohesion (thus mechanical/dimensional stability) of the overall core composite. In addition to bridging the particles, the polymeric binder offers viscoelastic damping (thus acoustic energy absorption), leading to superior noise reduction. The overall system is lightweight and possesses fire/flame retardancy similar to conventional gypsum boards. Furthermore, the high insulation efficiency afforded by the large void fraction protects the framing structure (2″×4″ studs) from becoming overheated in the event of an actual fire. The strengthening of the gypsum wallboard products made with the low-density cores of the '572 Application is primarily attributed to the strength of cover paper or other higher density layers formed at the surfaces of the wallboard, and the core itself is reduced in weight.
A longstanding need exists in the industry to substantially enhance the strength-to-weight ratio of cementitious materials, including cement and wallboard products, to produce lightweight products or stronger, wear resistant products. In addition, eliminating or reducing other additives, such as wallboard cover paper and glass fiber can reduce board and construction costs, environmental degradation and hazards to human health. Furthermore, a need exists to improve the thermal and sound insulation properties of high strength cementitious building materials.